The present invention relates to apparatus and methods for signal modulation. More specifically, this invention relates to apparatus and methods for low-voltage, current-folded signal modulator circuits.
A signal modulator circuit multiplies a first signal by a second signal, and may be used to shift the frequency components of the first signal from one frequency band to another frequency band. For example, a signal modulator circuit may be used to modulate a low frequency baseband signal (the first signal) by a high frequency carrier signal (the second signal), such as modulating a telephone signal that has frequency components from approximately 300-3400 Hz onto a higher frequency carrier signal (e.g. a 2 MHZ carrier signal). A modulator also may be used to shift a low frequency voice signal to a high frequency radio carrier signal for transmission over long distances. Modulating a signal onto a higher frequency carrier signal is called up-conversion. A signal modulator also may be used to down-convert a signal to lower frequencies.
One previously known signal modulator circuit 10, commonly called a Gilbert-cell mixer, is shown in schematic diagram form in FIG. 1. Gilbert-cell mixer 10 includes transconductance amplifier 12 and cross-coupled differential pair 14. Gilbert-cell mixer 10 receives differential first signal (V.sub.IN.sup.+ -V.sub.IN.sup.-) and second signal (V.sub.LO.sup.+ -V.sub.LO.sup.-), and provides differential output signal (V.sub.OUT.sup.+ -V.sub.OUT.sup.-). First signal (V.sub.IN.sup.+ -V.sub.IN.sup.-) may be a baseband signal, and second signal (V.sub.LO.sup.+ -V.sub.LO.sup.-) may be a high frequency modulation signal generated by a local oscillator. Output signal (V.sub.OUT.sup.+ -V.sub.OUT.sup.-) is the modulated output.
Transconductance amplifier 12 includes current source 11, emitter resistors 13A and 13B, and transistors 15 and 16. Cross-coupled differential pair 14 includes differential pair transistors 17 and 18, differential pair transistors 19 and 20, and resistors 21 and 22. Differential pair transistors 17 and 18 are cross coupled with differential pair transistors 19 and 20.
Transistor 15 has a collector coupled to emitters of transistors 17 and 18, a base coupled to input V.sub.IN.sup.+, and an emitter coupled to a first terminal of resistor 13A. Transistor 16 has a collector coupled to emitters of transistors 19 and 20, a base coupled to input V.sub.IN.sup.-, and an emitter coupled to a first terminal resistor 13B. The second terminals of resistors 13A and 13B are coupled to GROUND through current source 11. Transistor 17 has a base coupled to input V.sub.LO.sup.+ and a collector coupled to supply voltage V.sub.CC through resistor 21. Transistor 18 has a base coupled to input V.sub.LO.sup.-, and a collector coupled to supply voltage V.sub.CC through resistor 22. Transistor 19 has a base coupled to input V.sub.LO.sup.-, and a collector coupled to supply voltage V.sub.CC through resistor 21. Transistor 20 has a base coupled to input V.sub.LO.sup.+, and a collector coupled to supply voltage V.sub.CC through resistor 22. Modulated output signals V.sub.OUT.sup.+ and V.sub.OUT.sup.- are provided at the collectors of transistors 17 and 20, respectively. Transconductance amplifier 12 converts differential first signal (V.sub.IN.sup.+ -V.sub.IN.sup.-) into differential current signal I.sub.X =(I.sub.X.sup.+ -I.sub.X.sup.-). Cross-coupled differential pair circuit 14 modulates differential current signal (I.sub.X.sup.+ -I.sub.X.sup.-) by second signal (V.sub.LO.sup.+ -V.sub.LO.sup.-) to produce differential output signal (V.sub.OUT.sup.+ -V.sub.OUT.sup.-)
For many applications (e.g. battery powered cellular telephones), it is desirable to implement a signal modulator that consumes as little power as possible, thereby minimizing its energy needs. The power consumption of a signal modulator circuit is proportional to the supply voltage used to power the circuit. Thus, using a lower supply voltage advantageously reduces the power consumption of the circuit. There are, however, inherent constraints in the Gilbert-cell mixer that set a lower limit on the supply voltage for the circuit.
For example, in circuit 10, supply voltage V.sub.CC may be expressed as: EQU V.sub.CC =V.sub.R-21 +V.sub.CE-17 +V.sub.CE-15 +V.sub.R-13A +V.sub.I-11 (1)
where V.sub.R-21 is the voltage drop across resistor 21, V.sub.CE-17 is collector-emitter voltage of transistor 17, V.sub.CE-15 is the collector-emitter voltage of transistor 15, V.sub.R-13A is the voltage drop across resistor 13A, and V.sub.I-11 is the voltage drop across current source 11. Transistors 17 and 15 enter saturation when their collector-emitter voltage drops below V.sub.CE-SAT (e.g., 0.4 volts). If V.sub.IN.sup.+ has a DC voltage of 1.4 volts and a voltage swing of .+-.0.25 (i.e., V.sub.IN.sup.+ has a maximum value of 1.65 volts and a minimum value of 1.15 volts), V.sub.CE-15 should be greater than V.sub.CE-SAT plus the voltage swing of V.sub.IN.sup.+. Therefore, V.sub.CE-15 is at least 0.65 volts to prevent transistor 15 from entering saturation and causing distortion in V.sub.OUT. If V.sub.LO.sup.+ has a voltage swing of 200 mV, then V.sub.CE-17 should be at least 0.60 volts (i.e., V.sub.CE-SAT +200 mV) to prevent transistor 17 from entering saturation. V.sub.R-21 may be, for example, 0.5 volts; V.sub.I-11 is typically 0.4-0.6 volts; and V.sub.R-13A equals the voltage swing in V.sub.IN.sup.+ (e.g., 0.25 volts). For these exemplary signal values, V.sub.CC must be at least 2.4-2.6 volts.
If a lower supply voltage is used, output signals V.sub.OUT.sup.+ and V.sub.OUT.sup.- may not have sufficient room to reach their peak amplitude. Also, a low supply voltage may cause transistors in circuit 10 to saturate, resulting in a non-linear output response that causes distortion in output signal (V.sub.OUT.sup.+ -V.sub.OUT.sup.-. With a low supply voltage, transistors in circuit 10 saturate for large values of V.sub.IN.sup.+ and V.sub.IN.sup.-. Thus, lowering the supply voltage results in a trade-off: the more that the supply voltage is lowered to save power, the more distortion may be present in the output signal. Therefore, the peak amplitude of V.sub.IN.sup.+ and V.sub.IN.sup.- and maximum distortion requirements in output signals V.sub.OUT.sup.+ and V.sub.OUT.sup.- are constraints that set a lower limit on the supply voltage of circuit 10.
It would, however, be desirable to provide signal modulator circuits that consume less power than previously known Gilbert-cell mixer circuits, such as circuit 10. In particular, it would be desirable to provide signal modulator circuits that consume low power by operating at a low supply voltage.
It also would be desirable to provide signal modulator circuits that produce an output signal with reduced distortion at low supply voltages.
It further would be desirable to provide signal modulator circuits that allow for greater input voltage swings at low supply voltages.